Styrene and acrylic acid copolymers of low molecular weight have found wide acceptance in industry. Such copolymers, sometimes denominated as "solution polymers" have been employed in inks, paper coatings and paints. Typically vinylic monomers have been produced using mass polymerization techniques in the presence of a solvent. Often, the reaction is expedited with the assistance of catalytic initiators and chain transfer agents, to control the molecular weight of the product copolymers. Examples of such copolymerization techniques are disclosed in U.S. Pat. Nos. 2,556,488; 3,673,168; 3,753,958; 3,879,357 and 3,968,059.
With the cost of petroleum feedstock rising in recent years, the price of vinyl monomers has risen as well. The solvents normally employed in solution polymerization are expensive and safety problems have arisen with use of volatile organic solvents. Since solution polymerization is typically carried out in large batches, if the process malfunctions, then an unacceptably large amount of product must be wasted.
Accordingly, a continuous bulk polymerization process, capable of selectively producing low molecular weight vinylic copolymers having a low dispersion index, employing thermal initiators, without the need of a solvent, catalyst or weight retarding agent. U.S. Pat. No. 4,414,370 teaches a bulk polymerization process for preparing low molecular weight vinylic polymers having a dispersion index less than 2, a number average molecular weight from about 1000 to 6000, and a solids content of from 80 to 95 percent by weight of a polymer.
A substantial amount of research effort in the automotive industry has been directed to the development of clear coat automotive finishes. Due to stringent air pollution regulations, high solids coating compositions have been formulated which contain substantially less solvent and more film forming binder. However, finishes resulting from these compositions have a significant number of craters which give the finish an unacceptable appearance for use on automobiles and trucks. Cratering is the formation of circular depressions on the coating surface. These depressions can vary between shallow dimples and deep holes that go through the coating to the next substrate. In severe cases, the coating applicator can find hundreds of such defects on the painted object requiring repair or rejection of that object.
Heretofore, the problem of cratering defects have been attacked by use of additives to high solids coating compositions. Such additives are shown, for example, by U.S. Pat. Nos. 4,371,657; 4,396,680; and 4,402,983. Such additives, however, may also result in a worsening of appearance while decreasing cratering.
The novel process of the present invention produces polymers solutions which have greater crater resistance, reduced solution viscosity and improved durability than polymers produced by conventional techniques without the use of additives.